The Out-only converters shouldn't need to implement loadFile(), as it
shouldn't be called - a converter is not used for input unless it says
it supports input. At the same time, provide the "ground state"
implementations for optionsHelp(), outputOptions() and probeFile() to
save the trivial implementations the need to duplicate one another.
In the process, make the handling of loadFile()'s outputConverter more
consistent among those that do implement it. Always set
outputConverter if it's initially null (the caller does assert this).
Pick-to: 6.6 6.5
Task-number: QTBUG-111228
Change-Id: I856d12c791d1f8e0accdb7dd1412d493117b2302
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
1022b0d111